1. Field of the Invention
Embodiments of the present invention generally relate to a system and process for forming a patterned layer on desired regions of a surface of a substrate.
2. Description of the Related Art
Solar cells are photovoltaic (PV) devices that convert sunlight directly into electrical power. The PV market has experienced growth at annual rates exceeding 30% for the last ten years. Some articles suggest that solar cell power production world-wide may exceed 10 GWp in the near future. It is estimated that more than 95% of all solar modules are silicon wafer based. The high market growth rate in combination with the need to substantially reduce solar electricity costs has resulted in a number of serious challenges for inexpensively forming high quality solar cells. Therefore, one major component in making commercially viable solar cells lies in reducing the manufacturing costs required to form the solar cells by improving the device yield and increasing the substrate throughput.
Solar cells typically have one or more p-n junctions. Each p-n junction comprises two different regions within a semiconductor material where one side is denoted as the p-type region and the other as the n-type region. When the p-n junction of a solar cell is exposed to sunlight (consisting of energy from photons), the sunlight is directly converted to electricity through the PV effect. Solar cells generate a specific amount of electric power and are tiled into modules sized to deliver the desired amount of system power. Solar modules are joined into panels with specific frames and connectors. Solar cells are commonly formed on silicon substrates, which may be single or multicrystalline silicon substrates. A typical solar cell includes a silicon wafer, substrate, or sheet typically less than about 0.3 mm thick with a thin layer of n-type silicon on top of a p-type region formed on the substrate.
FIGS. 1A and 1B schematically depicts a standard silicon solar cell 10 fabricated on a wafer 11. The wafer 11 includes a p-type base region 21, an n-type emitter region 22, and a p-n junction region 23 disposed therebetween. An n-type region, or n-type semiconductor, is formed by doping the semiconductor with certain types of elements (e.g., phosphorus (P), arsenic (As), or antimony (Sb)) in order to increase the number of negative charge carriers, i.e., electrons. Similarly, a p-type region, or p-type semiconductor, is formed by the addition of trivalent atoms to the crystal lattice, resulting in a missing electron from one of the four covalent bonds normal for the silicon lattice. Thus the dopant atom can accept an electron from a neighboring atoms covalent bond to complete the fourth bond. The dopant atom accepts an electron, causing the loss of half of one bond from the neighboring atom and resulting in the formation of a “hole”.
When light falls on the solar cell, energy from the incident photons generates electron-hole pairs on both sides of the p-n junction region 13. Electrons diffuse across the p-n junction to a lower energy level and holes diffuse in the opposite direction, creating a negative charge on the emitter and a corresponding positive charge builds up in the base. When an electrical circuit is made between the emitter and the base and the p-n junction is exposed to certain wavelengths of light, a current will flow. The electrical current generated by the semiconductor when illuminated flows through contacts disposed on the frontside 18, i.e. the light-receiving side, and the backside 19 of the solar cell 10. The top contact structure, as shown in FIG. 1A, is generally configured as widely-spaced thin metal lines, or fingers 14, that supply current to a larger bus bar 15. The back contact 25 is generally not constrained to be formed in multiple thin metal lines, since it does not prevent incident light from striking solar cell 10. Solar cell 10 is generally covered with a thin layer of dielectric material, such as Si3N4, to act as an anti-reflection coating 16, or ARC, to minimize light reflection from the top surface 22A of solar cell 10.
Screen printing has long been used in printing designs on objects, such as cloth or ceramics, and is used in the electronics industry for printing electrical component designs, such as electrical contacts or interconnects on the surface of a substrate. State of the art solar cell fabrication processes also use screen printing processes. In some applications, it is desirable to screen print contact lines, such as fingers 14, on the solar cell substrate. The fingers 14 are in contact with the substrate are adapted to form an Ohmic connection with one or more doped regions (e.g., n-type emitter region 22). An Ohmic contact is a region on a semiconductor device that has been prepared so that the current-voltage (I-V) curve of the device is linear and symmetric, i.e., there is no high resistance interface between the doped silicon region of the semiconductor device and the metal contact. Low-resistance, stable contacts are critical for the performance of the solar cell and reliability of the circuits formed in the solar cell fabrication process. To enhance the contact with the solar cell device it is typical to position a finger 14 on a heavily doped region 17 formed within the substrate surface to enable the formation of an Ohmic contact. Since the formed heavily doped regions 17, due to their electrical properties, tend to block or minimize the amount light that can pass there through it is desirable to minimize their size, while also making these regions large enough to assure that the fingers 14 can be reliably aligned and formed thereon. The misalignment of the deposited fingers 14 to the underlying heavily doped regions 17 due to errors in the positioning of the substrate on an automated transferring device, defects in the edge of the substrate, unknown registration and alignment of the heavily doped region 17 on the substrate surface and/or shifting of the substrate on the automated transferring device can lead to poor device performance and low device efficiency. Heavily doped regions 17 may be formed on the substrate surface using a variety of patterning techniques to create areas of heavier and lighter doping, for example by performing phosphorus diffusion steps using a patterned diffusion barrier. A backside contact completes the electrical circuit required for solar cell to produce a current by forming an Ohmic contact with p-type base region of the substrate.
Therefore, there is a need for a screen printing apparatus for the production of solar cells, electronic circuits, or other useful devices that has an improved method of controlling the alignment of the deposited metal feature(s) (e.g., fingers 14) to a heavily doped region using a screen printing or other similar process.